The present invention relates generally to rotary drum filters used in the pulp and papermaking industry for forming a mat of pulp fibers from a pulp slurry and for extracting liquid from the mat. More particularly, the present invention relates to shower pipes used in conjunction with such filters for washing chemicals from the mat after it is formed on the drum. More specifically, the invention relates to the liquid diffusion flanges of such shower pipes.
Rotary drum filters are well known and are commonly used, for example, in the pulp- and paper-making industry for separating wood pulp from pulp slurry. Such filters include a rotary drum partially submerged in a tank of pulp slurry. The drum surface is conventionally covered by a filter screen. As the screen rotates through the pulp slurry, a vacuum is applied within a portion of the drum, collecting a wet mat of fibers from the slurry on the screen. As the screen emerges from the tank, slurry liquid or filtrate is drawn inwardly through the screen by the vacuum and discharged through suitable piping, thereby removing the liquid from the mat. Examples of such filters are disclosed generally in U.S. Pat. No. 4,276,169 to Browne, et al., U.S. Pat. No. 4,248,716 to LaValley, U.S. Pat. No. 4,370,231 to LaValley, and in the patents cited therein.
Conventionally, about half a dozen parallel shower pipes are angularly spaced around a segment of the drum extending from just above the surface of the slurry to the top of the drum, proceeding in the direction of drum rotation. These pipes extend axially of the drum and are supported adjacent the ends of the drum. Washing liquid is discharged in a spray from the shower pipes to wash the pulp fibers as the mat emerges from the slurry. The pipes are spaced a fixed radial distance from the filter screen. This distance is preferably constant along the lengths of the pipes so that spray intensity and distribution are substantially uniform all the way across the mat. Examples of such shower pipes are disclosed in U.S. Pat. Nos. 3,150,082 to Rich, 3,363,774 to Luthi, and 4,522,716 to LaValley.
In applicant's aforementioned prior U.S. Pat. No. 4,522,716 and copending application Ser. Nos. 703,033, and 709,450, there are disclosed improved shower pipe designs and methods for their manufacture which are directed to solving the problems of shower pipe corrosion and sag, and of providing complete spray coverage without overconcentration to an extent such that the spray tears the mat. The shower pipe improvements of such prior patent and applications are, for the most part, incorporated in the shower pipes of the present invention, and therefore the disclosures of such prior patent and applications are incorporated herein by reference.
One problem which the shower pipes of the aforementioned patent and applications do not address is the problem of heat loss from the washing liquid from the time of its emission from the shower pipes until it impinges the mat on the drum surface. It is important that the washing liquid be within a certain high-temperature range when it contacts the mat. Accordingly, the amount of heat loss from the spray liquid as it travels between the pipes and the mat determines to what temperature the liquid must be heated as it leaves the pipes. This, in turn, determines how much energy must be expended in maintaining the required temperature of the liquid.
Typically, shower pipes are spaced a radial distance of at least several inches from the surface of the filter drum, and the diffusion flanges of such pipes also terminate a substantial distance from such surface. Consequently, a substantial heat loss from the liquid occurs in traveling from the pipes to the drum. Therefore, substantial amounts of energy and therefore money, are expended in heating the washing liquid to a temperature that will compensate for such heat loss.
A second problem which the shower pipes of applicant's aforementioned prior patent and applications does not address is that of mat "roll-up" on the drum, or excessive mat thickness. During formation of the pulp mat on the drum surface in the upper reaches of the vat of pulp slurry as vacuum is applied to such surface, the mat often rolls back on itself into a roll or slug of stock of excessive thickness. This so-called "roll-up" sometimes emerges from the vat, carried on the upwardly rotating drum surface. If this occurs with a roll-up of sufficient diameter, the roll-up becomes wedged between the drum and the lowermost shower pipe of the bank of such pipes arranged along the emerging side of the drum surface, sometimes permanently deforming or even breaking the pipe or its diffusion flanges to necessitate their replacement, interfering with desired spray diffusion patterns, and interrupting proper mat formation.
A third problem not addressed by the shower pipes of applicant's aforementioned prior patent and applications is that of achieving the desired spray diffusion pattern which does not overconcentrate the spray at any given position on the mat and yet provides complete spray coverage with a shower pipe having only a single row of spray holes arranged axially of the pipe. Heretofore it has been necessary to provide a shower pipe with at least two circumstantially spaced rows of spray holes with the holes of one row being staggered with respect to the holes of the adjacent row to achieve the desired spray pattern. If the same spray diffusion pattern can be provided with a single row of holes, shower pipe manufacturing costs can be reduced.
Accordingly, there is a need for a shower pipe which has all of the attributes of the shower pipes of applicant's aforementioned prior patent and applications, and yet which alleviates the aforementioned problems associated with known shower pipe designs.
Objectives of the present invention, therefore, are to provide a shower pipe having the attributes of the shower pipes described in the aforementioned U.S. Pat. No. 4,522,716 and application Ser. Nos. 703,033 and 709,450, and in addition, features which:
1. inhibit heat loss from the washing liquid as it travels between the shower pipes and the mat;
2. prevent or inhibit pulp stock roll-up or other excessive stock thickness on the surface of the filter drum beyond the lowermost shower pipe;
3.prevent or inhibit shower pipe damage caused by roll-up; and
4. provide a desired spray diffusion pattern from a shower pipe having only a single row of spray holes.